1. Field of the Invention
The present invention relates to a method of generating color separation data and an image processing apparatus, and specifically, to generation of color separation data in the case that printing materials of particular colors are used for a printing apparatus.
2. Description of the Related Art
If printing materials used in a printing apparatus are, for example, cyan, magenta, yellow, and black inks or toners, then to generate print data, image processing is carried out to convert image data for R, G, and B into color separation data for C, M, Y, and K corresponding to the respective printing materials. The conversion process is normally executed using a look up table (LUT). Specifically, the contents of the table specify how to generate color separation data, that is, which combination of printing materials are to be used for a certain combination of R, G, and B data.
In the field of printing using inks or toners, it is known to use printing materials of what are called particular colors such as red, orange, green, and blue in addition to printing materials of cyan, magenta, yellow, and black, which are normally used, in order to expand the color gamut that can be reproduced by a printing apparatus, for example, to allow colors of a higher saturation to be reproduced.
Japanese Patent Application Laid-Open No. 2000-32284 describes a conventional example of a method of generating color separation data when such particular colors are used. In this case, color separation data is generated so that the gamut of the printing apparatus is reproduced using black and inks of two or three other colors including particular colors. For example, as shown in FIG. 68, for an a*b* plane in an L*a*b* space, color separation data (M, R, and K) and (Y, R, and K) is generated so as to reproduce a gamut k bounded by the hue M of a magenta ink and the hue R of a red ink and a gamut 1 bounded by the hue Y of a yellow ink and the hue R, using a combination of magenta, red, and black inks and a combination of yellow, red, and black inks, respectively. This document uses such combinations of color separation data to reduce the number of color pieces that need to be subjected to colorimetry in order to create an LUT. This also makes it possible to reduce the maximum amount of printing materials used in the printing apparatus.
However, the generation of color separation data described in Japanese Patent Application Laid-Open No. 2000-32284 simply reproduces the gamut of a saturation higher than that achieved by a printing material system not using any particular colors, by using, for a particular color printing material, the hue of this printing material and hues which are close to this hue and which are located on its respective sides (along a hue coordinate in color space). That is, the particular color printing material can realize a higher saturation or lightness than the basic-color printing materials of yellow, magenta, and cyan (and black) that are normally used. However, this method gives no advantages other than being suitable for reproduction of high saturations or the like with the limited hues mentioned above.
The inventor has made the present invention by attempting not only to use the particular color printing materials with the particular hues to obtain a high saturation as described above but also effectively to utilize the particularly high saturation and lightness of the particular color printing materials to expand the entire reproducible gamut.